16 FOUNDATIONS OF BIOLOGY 



finally, with their energy content nearly or entirely ex- 

 hausted, are eliminated as EXCRETIONS. This continual 

 waste must, if life is to persist, be counterbalanced by a 

 proportionate intake of food in order to renew the supply of 

 energy and afford the materials which, after preliminary 

 changes, are made into an integral part of the living organ- 

 ism. Thus in living the animal or plant is partially consum- 

 ing and rebuilding itself continually. This dual process is 

 METABOLISM. When constructive metabolism. ANABOLISM^ 

 keeps pace with destructive metabolism. KATABOLISM^ the 

 individual remains essentially unchanged and this is the 

 normal condition of adult life. During youth the anabolic 

 phases are in the ascendency and growth occurs, while old 

 age is characterized by a predominance of katabolic processes. 



3. Growth 



The results of metabolism force themselves upon our 

 attention chiefly as growth, or permanent increase in 

 the size of the individual. As a rule growth in plants con- 

 tinues more or less rapidly throughout life, while in 

 animals it is confined mainly to the early part of the 

 individual's existence, or youth. Indeed, at birth a child 

 is about a billion times larger than the egg from which it 

 has developed. 



Growth means that the organism makes over the materials 

 which it receives in the form of food from its environment 

 and fits them into the protoplasmic organization here and 

 there throughout as needed. This method 'of addition of 

 materials, which is termed growth by INTUSSUSCEPTION, is 

 highly characteristic of life. When growth occurs in the 

 non-living world, it is typically by accretion; as, for example, 

 in crystals where new material of the same kind is superim- 

 posed upon the surface. But protoplasm, with materials 



